The difference between the highest bid price and lowest ask price for a security, representing the transaction cost of trading and a measure of liquidity.
The bid-ask spread is the gap between the highest price a buyer is willing to pay (Bid Price) and the lowest price a seller is willing to accept (Ask Price). It represents the implicit transaction cost of trading — you buy at the ask and can only immediately sell at the lower bid, so you start every trade slightly underwater by the spread amount.
In Indian markets, spread varies dramatically by stock liquidity. Nifty 50 components like HDFC Bank or Infosys typically have spreads of Rs 0.05-0.20 (less than 0.01% of price) during market hours. Mid-cap stocks may have spreads of Rs 0.50-2.00 (0.05-0.20%). Small-cap and micro-cap stocks can have spreads of Rs 5-20 (1-3% of price), making frequent trading in these names extremely expensive.
Market makers and high-frequency traders profit by capturing the spread — buying at the bid and selling at the ask repeatedly. On the NSE, the exchange incentivises liquidity provision through lower transaction charges for market-making programmes in less liquid securities. This helps tighten spreads and improve price discovery.
Spreads widen during periods of uncertainty: market open and close, during major news events (RBI policy, Union Budget, quarterly results), and in pre-open and post-close sessions. A stock that normally has a Rs 0.10 spread might see it widen to Rs 1-2 during its quarterly results announcement as market participants reprice the stock. Similarly, spreads widen dramatically during Circuit Breaker events or market-wide volatility spikes.
For long-term investors, the bid-ask spread is a minor consideration. But for Day Trading and scalping strategies, it is a critical factor. If you trade a stock with a Rs 1 spread and make 10 round trips per day, you pay Rs 20 per share in spread costs alone — before brokerage and taxes. This is why active traders overwhelmingly prefer large-cap, high-liquidity names.